This invention relates to a class of compounds useful for imparting photochromic behavior to optically clear plastic lenses such as sunglass lenses. More particularly, it relates to a novel class of asymmetrical photochromic mercury complexes of diarylthiocarbazones.
Photochromic compounds become colored or change color upon being exposed to actinic electromagnetic radiation and then revert to their original color when the radiation is removed. Thus, an ophthalmic lens containing an operative photochromic compound has a luminous transmittance and/or color which varies depending on the intensity of ambient light. At present, it is believed that the only commercially successful photochromic lens comprises a glass matrix having fine silver halide particles dispersed therein. Lenses of this type are available under the trademark "Photogray" and have become increasingly popular since their introduction about 10 years ago.
In recent years, several ophthalmic quality plastic materials have been substituted for glass as a lens material. These too are becoming increasingly popular, and ophthalmic quality lenses of, for example, allyl diglycol carbonate, cellulose acetate Butyrate, cellulose acetate, polymethacylate, polymethylmethacrylate, and polycarbonate have become available. However, at the present time there are no commercially successful photochromic plastic lenses.
Since it is relatively easy to incorporate an organic compound into a plastic and since there are numerous known organic photochromic compounds, in general, efforts have been directed to the discovery or selection of an organic photochrome that can be incorporated into optically clear plastics to form a photochromic sunglass lens. However, despite the very large number of known photochromic compounds, a suitable photochrome is at this time not available. The most troublesome problem is the photolabile nature of the vast majority of the known photochromes. Thus, while many substances have been observed to undergo the photochromic process by darkening or changing color upon activation with incident radiation, and reverting toward their original color when the radiation is removed, the sensitivity and responsiveness to light of the vast majority of these compounds decreases with each cycle. This results in photochromic fatigue (light fatigue) and a relatively short useful life span of the photochrome.
The metal complexes of the diarylthiocarbazones, as a class, represent perhaps the most stable of all known organic photochromic compounds. The generic class of the compounds of this type are disclosed in U.S. Pat. No. 3,361,706 to Meriwether et al. However, the vast majority of the compounds falling within the class described by Meriwether are not suitable for the manufacture of a photochromic lens for use in snglasses or ski goggles.
This is because a successful photochrome must be characterized by a number of critical properties. Thus, to have optimal utility, an organic diarylthiocarbazono-metal complex must have an acceptable color in the unactivated state. In this regard, certain colors, e.g., orange, are not accepted by the public. Thus, a photochromic sunglass lens which is orange in either its activated or unactivated state would have no significant commercial impact. The rate of color change of a photochrome when exposed to radiation should be as high as possible. Obviously, a photochromic lens which takes hours or more to change color would have severely limited utility, as would a photochromic material which, for example, had to be placed in the dark before it reverted to its original unactivated state. The Meriwether patent mentioned above is helpful in this regard insofar as it teaches a method of accelerating the color changes of the diarylthiocarbazono-metal complexes.
Still another important property of the photochromic compound selected for use in sunglass lenses, as indicated above, is its photolability. Although the diarylthiocarbazonates are better than most other classes of photochromic compounds in this regard, most members of the class are not stable enough to continue to undergo activation and deactivation for a reasonable period of time.
In addition to these requirements, a successful photochromic compound must be suitable for incorporation into an optically clear plastic to produce a lens having a suitable light transmittance in both the activated and unactivated state. In this regard, it is obvious that photochromes which only marginally decrease the photo-transmittance on activation, or which absorb too much light, would have little utility.
Of the various possible metallic ions which are useable in the diarylthiocarbazone photochromic metal complexes, it has been discovered that mercury is by far the most suitable. However, the vast majority of these compounds absorb light at or above 485 nm in the unactivated state. This means that these photochromes have an orange to red hue which is clearly unacceptable for sunglass lenses. In fact, of the large number of mercury diarylthiocarbazone complexes known, it is believed that only three have absorption maxima below 485 nm, i.e., in the acceptable color range. These are phenyl mercury diphenylthiocarbazonate, which has an absorption maximum at about 475 nm, mercury bis [di (ortho) tolythiocarbazonate] which has an absorption maximum at 470 nm, and mercury bis [di (ortho-.alpha., .alpha., .alpha.-trifluoro) tolylthiocarbazonate], which has an absorption maximum at about 436 nm. However, the photostability of mercury bis [di (ortho) tolythiocarbazonate] is very poor. Accordingly, of all the mercury complexes of diarylthiocarbazones presently available, only phenyl mercury diphenylthiocarbazonate and mercury bis [di (ortho-.alpha., .alpha., .alpha.- trifluoro) tolythiocarbazonate] are correctly colored for use in sunglass lenses. The structural formulae of these latter two complexes are set forth below. ##STR1## The former compound is an example of a "mono" mercury complex, the latter of a "bis" complex.
A photochromic sunglass lens having sufficient fatigue resistance utilizing phenyl mercury diphenylthiocarbazonate as its photochrome is tan in the unactivated state and gray in the activated state. Its luminous transmittance in the unactivated state is about 35 to 40%. If, as is much preferred, a polarizing sheet is laminated into the photochromic sunglass lens, the luminous transmittance will be further reduced to about 14-16%. This rather low luminous transmittance in the unactivated state, particularly when a polarizing sheet is also included in the lens, is considered too low to enable construction of an ideal product.
Although the photochromic sunglass lens utilizing the mercury bis [di (ortho-.alpha., .alpha., .alpha.-trifluoro) tolylthiocarbazonate] photochrome is higher in the luminous transmittance in the unactivated state (48-64%), its rate of darkening is rather slow, eight minutes being required to reach 90% of the photo-stationary value. This photochromic lens is either tan or green in the unactivated state, depending on the amount of blue dye incorporated in the host. Accordingly, of all the known mercury diarylthiocarbazonates, this compound is perhaps best suited for sunglass applications.